City Research Online Visualization of Origins, Destinations and Flows with Od Maps

We present a new technique for the visual exploration of origins (O) and destinations (D) arranged in geographical space. Previous attempts to map the flows between origins and destinations have suffered from problems of occlusion usually requiring some form of generalisation, such as aggregation or flow density estimation before they can be visualized. This can lead to loss of detail or the introduction of arbitrary artifacts in the visual representation. Here, we propose mapping OD vectors as cells rather than lines, comparable with the process of constructing OD matrices, but unlike the OD matrix we preserve the spatial layout of all origin and destination locations by constructing a gridded two-level spatial treemap. The result is a set of spatially ordered small multiples upon which any arbitrary geographic data may be projected. Using a HashGrid spatial data structure we explore the characteristics of the technique through a software prototype that allows interactive query and visualisation of 105 to 106 simulated and recorded OD vectors. The technique is illustrated using US county to county migration and commuting statistics.